1. Field of the Invention
The invention relates to blades for windmills and turbomachines.
2. Discussion of Related Prior Art
Wind turbines are widely used to harness wind energy for beneficial use. Such uses include pumping water, running mechanical devices such as milling equipment, and in recent times creating electrical energy. Wind turbines generally fall in two categories: vertical axis and horizontal axis, and they typically comprise a plurality of blades attached to a hub that is attached to a shaft. As the wind contacts the blades, it causes the blades to rotate the shaft.
Much attention is given to the aerodynamic properties of turbomachine blades, and airfoil theory for the blades. U.S. Pat. No. 6,030,179 provides a detailed discussion of the significance of blade design as it relates to wind turbines and discloses a number of blade designs used in the art in attempts to achieve maximum efficiency for a given set of wind conditions.
Using atmospheric wind to generate power requires a method to spin an electrical generator with the maximum amount of torque and speed for a given wind speed. An asymmetrical airfoil is often selected for Horizontal Axis Wind Turbines (HAWT) because the lifting force can be used to drive a propeller. Through extensive previous research, it has been found that using airfoil lifting force achieves the greatest amount of power for a given wind speed, with the maximum amount possible governed by Betz Law. The same variables exist to determine the amount of lift for a wind turbine airfoil as an airplane wing. However a wing rotating about a fixed axis creates a range of velocities that increase along an airfoil from the base attached at the hub to the tip, where the blade velocity is at a maximum.
The relative velocity and angle of attack that a wind turbine blade experiences depends upon this velocity profile, wind speed, and blade geometry. These factors therefore determine the amount of lift that can be created and thereby the amount of power available to turn a generator.
The largest variable governing wind turbine blade design is the variation of wind speed. As stated previously, the amount of power available in the wind increases as the cube of the wind speed, so most designers disregard wind turbine output below 5-7 mph due to the low output. Most commercial wind turbines are designed to optimally perform in a range from 7 mph to 23 mph.
Even within this relatively small range of wind speeds, the variables affecting lift can change along the length of a wind turbine blade resulting in varying degrees of lift. For a fixed blade with an initial angle of attack of 10 degrees, the following chart shows the relative velocity a section of blade 24″ from the center of rotation experiences, along with the angle of attack:
ApparentAngle ofVwindVelocityattack(mph)(mph)(degrees)00.080.034.631.257.631.2721.29.31030.39.31236.39.31529.820.32039.720.32549.620.3These charts use an assumption of tip speed ratio (TSR) which is the speed of the blade tip versus the speed of the wind. It can be seen that a fixed blade will only achieve the ideal angle of attack (>0 degrees and <12 degrees) for a small range of wind speed.
One of the problems this creates is at lower speeds where the angle of attack is not great enough to create substantial lift. It's common for wind turbines to not turn at all at speeds less than 5 mph, and it requires a sustained amount of wind to turn fast enough to achieve an angle of attack resulting in lift. As wind power is becoming widely adopted, turbines are put into areas that have variable wind speeds or lower overall averages. A fixed blade will only generate power when there are sustained winds over a given wind speed, and it can turn fast enough to generate lift.
One approach today is to design a blade that has a steeper pitch in closer to the hub; this presents a better angle of attack at lower wind speeds and allows the turbine to start up faster, generating more overall power (see chart below for the same example at a pitch of 32 degrees).
ApparentAngle ofVwindVelocityattack(mph)(mph)(degrees)00.058.034.69.257.69.2721.2−12.71030.3−12.71236.3−12.71529.8−1.72039.7−1.72549.6−1.7This design approach gives rise to a compromise, as some amount of power is given up at higher speeds in order to get the turbine turning sooner. A common approach is to vary the angle of attack along the length of the blade, compromising peak efficiency for a wider range of usable wind speeds.
Large-scale commercial turbines are able to vary the blade pitch during operation, thereby achieving a high degree of efficiency for a very wide range of wind speeds. This requires complex mechanisms and controls, and is not currently used on small scale wind turbines.
Other aspects of the airfoil can be changed to maximize lift for a range of wind speeds. As stated before, increasing chord length is second variable that can achieve more lift for a given wind speed. There is a trade-off between chord length and increasing drag, which will slow a blade down. This effect is similar to trailing edge wing flaps on an airplane that deploy to increase lift and lower airspeeds. Due to the complexity of mechanisms on a wind turbine blade, this method is not currently used.
A third way to maximize lift is to increase the camber. This is the relative curvature of the airfoil shape, which affects the air speeds on the top and bottom. In general, increasing a wing's camber will increase lift up to a certain airspeed where drag becomes a greater factor.
Optimization of the various factors of a given blade design in the prior art results in a rigid or sculpted monolithic airfoil design, which results in added weight to the turbine. As a consequence, more energy is needed to turn the wind turbine. In addition, optimization is usually for a given range of atmospheric wind speed. If the atmospheric winds are outside of this range the wind turbine is out of optimization.
Rigid blades also are incapable of flexing to spill off wind in high wind situations, and are also incapable of flexing in reaction to turbulent winds, or winds passing in a non-laminar condition. Rigid blades also make windmills cumbersome and less portable and more difficult to set up.
Currently, there is a need for a blade design that can be manipulated to vary its cord length, camber and angle of attack along the length of the blade to have uniform or non-uniform characteristics based on wind conditions. Also, a need exists to have a blade design that can flex to the various forces of atmospheric winds.